The diazo process is well known for reproduction of engineering and technical drawings in which the image appears as blue lines on a white background. These drawings are often called "blue lines" and are the reverse in appearance from the older type of blueprint in which the image is in the form of white lines on a blue background. According to the usual diazo process, a master or original is exposed to ultraviolet light from a suitable source, the light being transmitted through the original onto the sensitized surface of a diazo copy paper. The opaque or darker portions of the original which constitute the image to be copied, do not transmit the light or transmit less of the light than the paper itself, to expose a pattern on the diazo paper corresponding to the original image. The paper is developed by well known dry processes by exposure to ammonia to render the image visible. Diazo copying is very widely employed since no dark room or wet procesing facilities are required.
The original from which diazo copies are made is usually a thin tracing paper or vellum on which the image is drawn or otherwise provided in black pencil or ink to provide a high contrast image from which prints can be made. The blue line print or copy can not itself be employed as an original from which to make a diazo copy, since such a copy will be unreadable or only marginally readable. The blue lines of a diazo copy are transmissive to the light from the illumination source and thus the image is not "seen" by the light. As a consequence, the light from the source passes through the blue image and exposes the diazo paper without delineating the image. Some absorption of light by the blue image can occur, but not sufficiently to yield a developed copy of practical readability.
The problem of making a diazo copy from a blue line original is also present in copying apparatus using reflected rather than transmitted light. In this instance, light from an illumination source striking the surface of a blue line original is reflected off the surface with little or no absorption by the blue image, such that the diazo copy paper is exposed without appearance of the image in any readily readable fashion.
Referring to FIG. 1, there is shown a plot of the spectral energy distribution of a typical diazo copier fluorescent lamp and of the spectral sensitivity of a typical diazo paper. The paper has its major sensitivity in the range of about 3400-4500 Angstroms, with a peak at about 4000 Angstroms. The lamp has its major energy distributed about a peak of about 4200 Angstroms, with secondary peaks at about 5500 and 5800 Angstroms. The blue image of a diazo print has an absorption range of about 4600-5900 Angstroms. The light from a conventional diazo illumination source is not to an appreciable extent responsive to the blue image of a diazo print and is transmitted through the image as if it were not present. In FIG. 1 it is seen that the major energy band of the lamp extends only slightly into the blue line absorption region and is at a relatively low energy level. The major energy of the lamp passes through the blue image and exposes the diazo paper without responding to the image. The lamp energy which is absorbed by the blue image is not effective to expose this image on the diazo paper, since the major exposure of the diazo paper occurs by the energy which has passed through the blue image; thus, no readable copy results.
It would be extremely beneficial to have the capability of making diazo copies from blue line prints without resort to the original. The original is not often available at an office or facility where a copy is desired, while the blue line copies are found almost universally in architectural and engineering offices, machine shops and other technical facilities. These ubiquitous blue line drawings are often in need to be copied without availability of or access to the original.